'Teen Mom' Mackenzie McKee Is Single And Pregnant ... With Estranged Hubby's Baby

'Teen Mom' star Mackenzie McKee is expecting her third baby with husband Josh, but she'll be raising the kid as a single mom because they've finally called it quits.

McKee barely showed during her 18 week ultrasound. She's expecting a boy -- the due date is September 7th.

She says she's no longer living with Josh, the father of their 2-year-old daughter, Jaxie, and 4-year-old son, Gannon.

It's not the first time they've called it quits ... Josh walked out last April, but clearly came back.

At least McKee's not alone ... she now has something in common with a Hollywood A-lister.